Seizure and movement disorders can be broadly characterised as those disorders which arise when the brain's electrical activity is periodically disturbed, resulting in some degree of temporary brain dysfunction. The boundaries between seizure disorders (such as epilepsy for example) and movement disorders are difficult to define; some syndromes or diseases may combine the two and many manifestations of one are similar to the other. Furthermore, the diagnosis of epilepsy for example can be indicative for the future development of a movement disorder. Despite that, from a clinical perspective, seizure and movement disorders are distinct from each other.
Epilepsies constitute a diverse collection of seizure disorders that affect about 3% of the population at some time in their lives. An epileptic seizure can be defined as an episodic change in behaviour caused by the disordered firing of populations of neurons in the central nervous system. This results in varying degrees of involuntary muscle contraction and often a loss of consciousness. Epilepsy syndromes have been classified into more than 40 distinct types based upon characteristic symptoms, types of seizure, cause, age of onset and Electroencephalography (EEG) patterns (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). However, the single feature that is common to all epileptic syndromes is the persistent increase in neuronal excitability that is both occasionally and unpredictably expressed as a seizure.
A genetic contribution to the aetiology of epilepsy has been estimated to be present in approximately 40% of affected individuals. As epileptic seizures may be the end-point of a number of molecular aberrations that ultimately disturb neuronal synchrony, the genetic basis for epilepsy is likely to be heterogeneous. There are over 200 Mendelian diseases which include epilepsy as part of the phenotype. In these diseases, seizures are symptomatic of underlying neurological involvement such as disturbances in brain structure or function. In contrast, there are also a number of “pure” epilepsy syndromes in which epilepsy is the sole manifestation in the affected individuals. These syndromes are termed idiopathic and account for over 60% of all epilepsy cases.
Idiopathic epilepsies have been further divided into partial and generalized sub-types. Partial (focal or local) epileptic fits arise from localized cortical discharges, so that only certain groups of muscles are involved and consciousness may be retained. However, in generalized epilepsy, EEG discharge shows no focus such that all subcortical regions of the brain are involved. Although the observation that generalized epilepsies are frequently inherited is understandable, the mechanism by which genetic defects, presumably expressed constitutively in the brain, give rise to partial seizures is less clear.
In neonates and infants, probably because brain myelination is incomplete, the distinction between partial and generalized epilepsies is less clear from clinical and neurobiological standpoints. Epilepsies in the first year of life were previously viewed as largely due to acquired perinatal factors. However, two benign autosomal dominant epilepsy syndromes are now well recognised in the first year of life. The first is benign familial neonatal epilepsy (BFNE) which usually presents around the third day of life and is characterised by tonic or clonic seizures. These seizures stop within a few weeks of age, with 5% of individuals having later febrile seizures and 11% later epilepsy. Studies have shown that the genetic basis for this syndrome in many cases is due to mutations in the potassium channel genes KCNQ2 and KCNQ3.
The second is benign familial infantile epilepsy (BFIE) which is an autosomal dominant seizure disorder of infancy in which seizure onset occurs at a mean age of 6 months with clusters of tonic or clonic partial or generalised seizures over a few days. Seizures are usually offset by around 2 years of age but it may be associated with paroxysmal dyskinesias (movement disorders) in later childhood in some individuals. Whilst no genes have been definitively identified to be causative of BFIE, a general linkage to chromosomes 19, 1 and 16 has previously been reported, with the vast majority of families showing linkage to the pericentromeric region of chromosome 16 at 16p11-16q12.1.
Movement disorders encompass a wide variety of neurological conditions affecting motor control and muscle tone. These conditions are typified by the inability to control certain bodily actions. Accordingly, these conditions pose a significant quality of life issue for patients. Nonlimiting examples of movement disorders include dyskinesias, Parkinson's syndrome, dystonias, myoclonus, chorea, tics, and tremor. Dystonia is a neurological disorder characterized by sustained, involuntary movements. These movements typically produce twisting postures. A large number of conditions produce dystonia, including genetic causes, toxin or drug-induced causes, and degenerative illnesses in which dystonia is manifested.
Essential tremor is another type of movement disorder, separate from dystonia, and is the most common cause of tremor in the adult population. Patients with essential tremor exhibit involuntary, rhythmic tremor, or shaking, of a body part. Commonly, essential tremor affects the hands, head, or voice, but it can also affect the tongue, legs, or trunk. In tasks which involve fine motor control, patients with essential tremor may have difficulty performing these skills. For example, a severe tremor in the hands makes eating, drinking, writing, and dressing, difficult. Whilst the exact cause of essential tremor is unknown, it is often inherited.
Yet another movement disorder is paroxysmal kinesigenic choreoathetosis (PKC), also known as paroxysmal kinesigenic dyskinesia (PKD). This condition is characterized by unilateral or bilateral involuntary movements precipitated by other sudden movements such as standing up from a sitting position, being startled, or changes in velocity; attacks include combinations of dystonia, choreoathetosis, and ballism, are sometimes preceded by an aura, and do not involve loss of consciousness. Attacks can be as frequent as 100 per day to as few as one per month. Attacks are usually a few seconds to five minutes in duration but can last several hours. Age of onset is typically in childhood and adolescence. Currently, the cause of familial PKC/PKD is unknown.
Infantile Convulsions and Choreoathetosis (ICCA) syndrome is a seizure and movement disorder. In ICCA, family members may have infantile seizures, PKC/PKD, or both. Families with ICCA or familial (autosomal dominant) PKC/PKD alone also show linkage to the large pericentromeric region of chromosome 16. The shared linkage region and co-occurrence of these disorders in families with ICCA has previously led to speculation that BFIE, PKC/PKD, and ICCA may be allelic.
There is a need for the identification of the causative gene(s) for the aforementioned disorders. Genes involved in these disorders will form the basis of diagnostic and therapeutic applications for patients with the disorders. This will enable proper management of affected individuals and will avoid over-investigation and over-treatment of patients.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Nucleotide and polypeptide sequences are referred to herein by a sequence identifier number (SEQ ID NO:). A summary of the sequence identifiers is provided in Table 1. A sequence listing is also provided at the end of the specification.
TABLE 1Summary of Sequence IdentifiersSequenceIdentifierDescriptionSEQ ID NO: 1PRRT2 frameshift mutation (c.629-630insC) - nucleotide sequenceSEQ ID NO: 2Amino acid sequence of c.629-630insC frameshift mutationSEQ ID NO: 3PRRT2 frameshift mutation (c.649-650insC) - nucleotide sequenceSEQ ID NO: 4Amino acid sequence of c.649-650insC frameshift mutationSEQ ID NO: 5PRRT2 splice site mutation (IVS2 + 1G > T) - nucleotide sequenceSEQ ID NO: 6PRRT2 splice site mutation (IVS2 + 5G > A) - nucleotide sequenceSEQ ID NO: 7PRRT2 missense mutation (c.950G > A) - nucleotide sequenceSEQ ID NO: 8Amino acid sequence of c.950G > A missense mutationSEQ ID NO: 9PRRT2 wild-type nucleotide sequence - coding regionSEQ ID NO: 10PRRT2 wild-type amino acid sequenceSEQ ID NO: 11PRRT2 wild-type nucleotide sequence - genomicSEQ ID NO: 12F Primer for PCR amplification of the PRRT2 c.649-650insC mutationSEQ ID NO: 13R Primer for PCR amplification of the PRRT2 c.649-650insC mutationSEQ ID NO: 14Sequence of PCR product amplified using the F and R primerSEQ ID NO: 15MLPA analysis - LPO Probe - PRRT2 exon 1SEQ ID NO: 16MLPA analysis - RPO Probe - PRRT2 exon 1SEQ ID NO: 17MLPA analysis - LPO Probe - PRRT2 exon 2-1SEQ ID NO: 18MLPA analysis - RPO Probe - PRRT2 exon 2-1SEQ ID NO: 19MLPA analysis - LPO Probe - PRRT2 exon 2-2SEQ ID NO: 20MLPA analysis - RPO Probe - PRRT2 exon 2-2SEQ ID NO: 21MLPA analysis - LPO Probe - PRRT2 exon 2-3SEQ ID NO: 22MLPA analysis - RPO Probe - PRRT2 exon 2-3SEQ ID NO: 23MLPA analysis - LPO Probe - PRRT2 exon 3SEQ ID NO: 24MLPA analysis - RPO Probe - PRRT2 exon 3SEQ ID NO: 25MLPA analysis - LPO Probe - PRRT2 exon 4SEQ ID NO: 26MLPA analysis - RPO Probe - PRRT2 exon 4